1. Field of the Invention
The present invention relates to a magnetic head testing apparatus for testing generally a magnetic head using a tunneling magnetoresistive effect element. In particular, it relates to a technique for non destructive inspection of a tunnel barrier layer of the element.
2. Description of the Related Art
In general, a magnetic head comprising separately a reading head and a writing head, mounted on the same slider is used in a disk drive. The reading head is a head dedicated for reading a data signal recorded magnetically on a disk medium.
Recently, a GMR (giant magnetoresistive effect) element (referred also as a GMR sensor) suited for achieving a high recording density of a disk medium is used for the reading head. However, as recent magnetic heads and the like are very a problem of sensitivity deterioration of the GMR element has grown in importance.
As a countermeasure for the problem, use of a novel tunneling magnetoresistive effect (TMR) element (referred to also as a TMR sensor) as the reading head has been proposed.
Unlike the GMR element, in a TMR element, the conducting direction of the sense electric current is in the vertical direction with respect to the film surface, such that the tunneling electric current flows in the tunnel barrier layer (for example, see U.S. Pat. No. 5,729,410 and the U.S. Pat. No. 5,898,547).
The characteristics of the TMR element depend largely on the quality of the tunnel barrier layer of the extremely thin film. Particularly in the case a pin hole is present in the tunnel barrier layer, the decline of the breakdown voltage is observed (for example, see Journal of Applied Physics, Vol. 91, No. 7, 2002; P. 4348-4352, “Dielectric breakdown in magnetic tunnel junctions having an ultrathin barrier” Bryan Oliver, Qing He, Xuefei Tang, and J. Nowark (Seagate Technology LLC.)).
The “breakdown voltage” refers to the critical voltage of causing an element destruction at the time of applying the electric current stress to the TMR element stepwise. Moreover, the “pin hole” refers to a part with a free layer and a pinned layer present on the both sides of the tunnel barrier layer, contacted locally with a metal due to the interface disturbance by the film formation irregularity of the tunnel barrier layer, the stress in the process and the like.
In the case a pin hole is present in the tunnel barrier layer, since the pin hole part has a low resistance, the electric current flowing in the tunnel barrier layer is concentrated. It is confirmed that the destructive damage to the tunnel barrier layer is accelerated by the local heat generating effect resulting from the electric current concentration. Therefore, for an improvement of the reliability of the reading head using the TMR element, it is essential to test for the presence of pin holes in the tunnel barrier layer.
In general, the presence of pin holes can be confirmed by the breakdown test. However, since the test is a destructive test, it cannot be used for the screening test for the reading head (magnetic head).